Figure 2.

Newly Developed Biomedical Applications of High-Field ¹⁹F NMR Spectroscopy: Analysis of Human Saliva and Tap Water. ¹⁹F NMR spectra of (a) an aqueous sodium fluoride calibration standard (δ_F = −119.5 ppm; final analyte concentration 17.6 µmol./L) and (b) a WMSS sample collected from a participant following an 8.00 h. oral abstention ‘fasting’ period. Analysis solutions contained 527 µL of a 20 µmol./L sodium fluoride solution, 60 µL of ²H₂O and 13 µL of a 50.00 mmol./L trifluoroacetate (TFA) internal standard (δ_F = −75.3 ppm). ¹⁹F NMR spectra were acquired on a JNM-ECZ600R/S1 600 MHz NMR spectrometer (operating at a frequency of 564.72 MHz for ¹⁹F), over a 400 ppm spectral width and with an FID acquisition time of 0.92 s. An 8.3 µs 90° pulse was used; the relaxation delay between pulses was 3 s. In total, 2048 scans were acquired with 256 K data points, which were then Fourier-transformed with zero-filling to 512 K data points and a single exponential function of 5.0 Hz. Baseline roll signal from the fast-relaxing fluoropolymer species in the NMR probe-head was removed using backward linear prediction (order = 16, sample data 512 points, reconstructed data 32 points); baseline correction was applied to spectra using a polynomial function. The resulting signal-to-noise (STN) ratio for the ¹⁹F resonance in (a) was 50:1. Chemical shift values were referenced to external fluorotrichloromethane (CFCl₃; Jeol UK Ltd. default reference setting). Inset: Partial ¹⁹F NMR spectrum of a local, East Midlands, UK, sample of tap water (shown in green) demonstrating the detection of fluoride anion therein; this spectrum was also acquired with backward linear prediction and 2048 scans.